They said they'd do it, and they did. Hundreds of rabbis either boycotted or stood as Republican presidential candidate Donald Trump began to speak at a pro-Israel conference and walked out to protest his "hate and bigotry" that hold troubling reminders for them of the Holocaust.

Yet, in a sign of the divisive power of the front-runner, hundreds of others in the 18,000-strong crowd applauded and loudly cheered Trump several times — particularly when he attacked President Barack Obama and Democratic presidential contender Hillary Clinton — and even gave him standing ovations. Obama "may be the worst thing ever to happen to Israel," he said during his 25-minute speech.

An Orthodox rabbi in a prayer shawl seated near the front was forced out by security moments after he shouted out once Trump began speaking in the Verizon Center in Washington DC where the conference for the powerful American Israel Public Affairs Committee lobby was held.

Rabbi Shmuel Herzfeld, leader of a prominent Washington DC Modern Orthodox synagogue, told the Washington Post that when Trump came to the podium, he immediately stood, put on his tallis — prayer shawl — and shouted: "Do not listen to this man. He is wicked. He inspires racists and bigots."

"With every cell in my body I felt the obligation as a rabbi to declare his wickedness to the world," Herzfeld said on Facebook later. Protesting rabbis and supporters went directly to other areas in the complex to pray, activists said.

Donald Trump protesters at AIPAC
Anti-Trump protesters gather outside the Verizon Center in Washington DC during the American Israel Public Affairs Committee Conference.Reuters

Trump has called himself "Israel's number one friend," though he has also said he would remain neutral in any peace talks between Israel and Palestine to be a more effective negotiator. But the rabbis who organised the boycott did so because they most oppose what they call Trump's "hate speech" reminiscent of Adolf Hitler's blaming the Jews for Germany's economic troubles before World War II.

Trump hasn't singled out Jews, but has blamed Muslims, Mexicans and other immigrants for America's economic woes.

"When he speaks hatefully of Mexicans or Muslims ... we recall a time when anti-Semitism put Jews at deathly danger," said a statement by boycott organisers from the Union for Reform Judaism, the largest Jewish group in the US.

While the Post called Trump's AIPAC speech "fiery," other media outlets called it restrained and predictable. He reiterated the Republican line on many issues important to Jews, saying that the Iran nuclear deal was "catrostrophic," the United Nations was an anti-Israel joke, and that Palestinians "glorify terrorists."

He closed his remarks by noting that his daughter, Ivanka, whose husband is an Orthodox Jew, had converted to Judaism and is expecting "a beautiful Jewish baby. In fact, it could be happening right now."

Rick Jacobs, the head of the Union for Reform Judaism, who called Trump a "person of hate and bigotry," said before the speech there are ongoing talks with the Trump campaign to arrange a meeting with candidate and rabbis and other leaders to discuss their objections to his statements.

He said that despite the walkout, he and other protestors would be attentive to Trump's message on Israel. But "even if we hear a good clear position, that doesn't erase his previous statements," he told Haaretz.